First, I will say this, if you are getting too much spam, esp from the same player or fc you can report them because it is considered harassment. Talked to a GM about this when I rolled a new char on a different server and was inundated with invites to a certain fc by different players, which I suspect were mostly the same person on different chars just inviting when they were on alts as well as their main.

As a fairly experienced guild leader in a few different games I must say that FCs who just send rando invites are generally the worst guilds to join. If you send a random invite or even a stock greeting + invite it tells me you are either a very inexperienced guild leader or a guild leader that doesn't care about their members because all they are to you are numbers. If the leader is inexperienced there is room for them to grow, but to me its not worth it to give the other ones numbers. Regardless the invite gets declined and that person gets blisted.